bpo-39481: Implementation for PEP 585 (#18239)

This implements things like `list[int]`,
which returns an object of type `types.GenericAlias`.
This object mostly acts as a proxy for `list`,
but has attributes `__origin__` and `__args__`
that allow recovering the parts (with values `list` and `(int,)`.

There is also an approximate notion of type variables;
e.g. `list[T]` has a `__parameters__` attribute equal to `(T,)`.
Type variables are objects of type `typing.TypeVar`.

1 files changed, 3 insertions, 0 deletions

diff --git a/Lib/types.py b/Lib/types.py
index ea3c0b29d5..ad2020ec69 100644
--- a/Lib/types.py
+++ b/Lib/types.py
@@ -293,4 +293,7 @@ def coroutine(func):
     return wrapped
 
 
+GenericAlias = type(list[int])
+
+
 __all__ = [n for n in globals() if n[:1] != '_']